Timing and Time Information Extraction from a Phase Modulated Signal in a Radio Controlled Clock Receiver

ABSTRACT

A system and method for a radio controlled clock receiver adapted to extract timing and time information from a phase modulated signal. The official time signal is broadcast from a central location using the modified modulation scheme of the present invention, which adds phase modulation that allows for greatly improved performance. The information modulated onto the phase contains a known synchronization sequence having good autocorrelation properties, error-correcting coding for the time information and notifications of daylight-saving-time (DST) transitions that are provided months in advance. The modulation scheme is based on a form of phase modulation, such as binary-phase-shift-keying (BPSK) or phase reversal keying (PRK). A superframe comprising multiple frames with repeated information allows for the accumulation of received energy over multiple frames to provide for a corresponding gain in the receiver.

REFERENCE TO PRIORITY APPLICATION

This application is a continuation of U.S. application Ser. No.13/345,084, filed Jan. 6, 2012, now U.S. Pat. No. 8,233,525, entitled“Timing and Time Information Extraction from a Phase Modulated Signal ina Radio Controlled Clock Receiver,” incorporated herein by reference inits entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under National Instituteof Standards and Technology SBIR Grant No. NB401000-11-04154. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the field of wireless communications,and more particularly relates to a radio controlled clock receiveradapted to extract timing and time information from a phase modulatedsignal.

BACKGROUND OF THE INVENTION

Radio-controlled-clock (RCC) devices that rely on time signal broadcastshave become widely used in recent years. A radio-controlled-clock (RCC)is a timekeeping device that provides the user with accurate timinginformation that is derived from a received signal, which is broadcastfrom a central location, to allow multiple users to be aligned orsynchronized in time. Colloquially, these are often referred to as“atomic clocks” due to the nature of the source used to derive thetiming at the broadcasting side. In the United States, the NationalInstitute of Standards and Technology (NIST) provides such broadcast inthe form of a low-frequency (60 kHz) digitally-modulated signal that istransmitted at high power from radio station WWVB in Fort Collins, Colo.The information encoded in this broadcast includes the official time ofthe United States. This also includes information regarding the timingof the implementation of daylight saving time (DST), which has changedin the United States over the years due to various considerations.

Reception of the time signal, however, is being challenged by a growingnumber of sources of electromagnetic interference. In particular, theon-frequency interference from the MSF radio station in the UnitedKingdom has been identified as a particularly challenging jammer forreceivers on the East Coast.

There is thus a need for a new protocol for time signal broadcasts, suchas that provided by WWVB, that attempts to cost-effectively address thereception challenges. Such a new protocol should preserve existingamplitude modulation properties of the transmitted signal, in order tomaintain backwards compatibility and not impact existing devices.

SUMMARY OF THE INVENTION

The present invention is a system and method for a radio controlledclock receiver adapted to extract timing and time information from aphase modulated signal. The system and method of the present inventionprovide a modified modulation scheme for transmission of the officialtime signal that is broadcast from a central location, and a receiveradapted to extract the timing and time information from this broadcast.The modified modulation scheme adds phase modulation that allows forgreatly improved performance. The information modulated onto the phasecontains a known synchronization sequence, error-correcting coding forthe time information and notifications of daylight-saving-time (DST)transitions that are provided months in advance.

The structure and method of operation of the receiver allows thetimekeeping functionality of a device to be accurate, reliable and powerefficient. The communication protocol of the present invention isadapted to allow prior-art devices to operate in accordance with thelegacy communication protocol such that they are unaffected by thechanges introduced to the protocol by the present invention, whereasdevices adapted to operate in accordance with the present inventionbenefit from various performance advantages. These advantages include(1) greater robustness of the communication link; (2) allowing reliableoperation at a much lower signal-to-noise-and-interference-ratio (SNIR);(3) greater reliability in providing the correct time; and (4) reducedenergy consumption which leads to extended battery life inbattery-operated devices.

In one embodiment of the present invention, the modulation applied tothe carrier is limited to its phase, thereby allowing existing devicesthat operate in accordance with the legacy communication protocol,whereby the information may be extracted through envelope detection, tocontinue to operate with the modified protocol without being affected.Although this backward compatibility property of the communicationprotocol of the present invention may represent a practical need whenupgrading an existing system, the scope of the invention is not limitedto the use of this modulation scheme and to operation in conjunctionwith an existing communication protocol.

The enhanced robustness offered by the present invention, resulting inreliable reception at lower SNIR values with respect to those requiredfor proper operation of prior art devices, is a result of the use of (1)a known synchronization sequence having good autocorrelation properties;(2) coding that allows for error detection and correction within thefields of information bits that are part of each data frame; and (3) theuse of a superior modulation scheme, such as binary-phase-shift-keying(BPSK) (also known as phase-reversal keying or PRK) in one embodiment ofthe present invention. The PRK modulation, representing an antipodalsystem, provides the largest distance in the signal space with respectto signal power, whereas the historical modulation schemes that are usedfor time broadcasting worldwide are based on pulse width modulation(PWM) that relies on amplitude demodulation, requiring a higher SNIR toachieve the same decision error probability or bit-error-rate (BER).

The enhanced reliability in assuming or setting the right time in adevice of the present invention may be partly achieved through the useof a time-computing procedure that considers not only the informationextracted from the received frame, but also the time that has beenassumed in the timekeeping device. For example, if the informationextracted from a received frame suggests that the year is many yearsahead of what the timekeeping device has been assuming for a long time,it is likely that the reception is in error and should be disregarded.

On a finer scale, when the correlation operation that makes use of theknown synchronization sequence in the received signal produces a noisyresult (i.e. the correlation peak is closer to the low-correlationresults), based on which the timing extraction may be inaccurate, thereceiver may apply averaging filtering, wherein the timing extractedfrom the received signal is weighted against the locally assumed time inthe device such that the timing adjustment considers them both insteadof being determined based solely on the received signal, as is typicallydone in existing prior art devices.

Furthermore, the system is scalable in that it allows for receiversexperiencing different reception conditions to use the received signaldifferently. In particular, it is designed to allow for the accumulationof received energy over multiple one-minute frames (i.e. throughout aone-hour superframe or a portion thereof), to provide for acorresponding gain in the receiver (e.g., reception for a whole hour mayprovide a gain of 60, or 18 dB, with respect to a single minute).

The features described supra serve to greatly increase the robustnessand reliability of the time signal communication system, allowing it tooperate at signal-to-noise ratios that are several orders of magnitudelower than those required in the existing scheme, while exhibiting evenhigher gains in scenarios of on-frequency jamming, to which the existingreceivers are particularly vulnerable.

There is thus provided in accordance with the invention, a radioreceiver comprising a receiver circuit operative to receive a phasemodulated (PM), pulse width modulation (PWM)/amplitude shift keyed (ASK)broadcast signal encoded with timing and time information, the timinginformation based on a known synchronization sequence and a circuitoperative to extract the timing and time information from the phase ofthe received signal.

There is also provided in accordance with the invention, a radioreceiver method, the method comprising receiving a phase modulated (PM),pulse width modulated (PWM)/amplitude shift keyed (ASK) broadcast signalencoded with timing and time information, the timing information basedon a known synchronization sequence and extracting the timing and timeinformation from the phase of the received signal.

There is further provided in accordance with the invention, a radioreceiver method for use in a timekeeping device, the method comprisingreceiving a phase modulated (PM), pulse width modulated (PWM)/amplitudeshift keyed (ASK) broadcast signal encoded with timing and timeinformation, the timing information based on a known synchronizationsequence, extracting the timing and time information from the phase ofthe received signal and correlating the timing information against aknown synchronization sequence so as to establish frame and symboltiming.

There is also provided in accordance with the invention, a radioreceiver method, the method comprising receiving a phase modulated (PM)broadcast signal encoded with timing and time information, wherein thetiming and time information, intended for synchronization and timereference purposes, is conveyed in the phase of the carrier portion ofthe broadcast signal and extracting the timing and time information fromthe phase of the received signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a high level block diagram illustrating an example timing andtime information transmitter of a system operating in accordance withthe present invention;

FIG. 2 is a high level block diagram illustrating an example timing andtime information receiver constructed in accordance with the presentinvention;

FIG. 3 is a diagram illustrating a first example pulse-width modulatedAM signal representing a ‘0’ bit;

FIG. 4 is a diagram illustrating a second example pulse-width modulatedAM signal representing a ‘0’ bit;

FIG. 5 is a diagram illustrating a first example pulse-width modulatedAM signal representing a ‘1’ bit;

FIG. 6 is a diagram illustrating a second example pulse-width modulatedAM signal representing a ‘1’ bit;

FIG. 7 is a diagram illustrating a first example pulse-width modulatedAM signal representing a marker ‘M’;

FIG. 8 is a diagram illustrating a second example pulse width modulatedAM signal representing a marker ‘M’;

FIG. 9 is a diagram illustrating the structure of an example data frameincorporating timing and time information;

FIG. 10 is a diagram illustrating an example embodiment of phasemodulation, shown at baseband, added to a pulse-width amplitudemodulated carrier;

FIG. 11 is a diagram illustrating the signal space representation of theprior art AM/pulse-width ‘0’ and ‘1’ signals, as well as that of the anexample embodiment of the present invention, where PRK is added onto theAM/pulse-width modulation;

FIG. 12 is a diagram illustrating an example receiver incorporating bothamplitude and phase modulation receiver paths;

FIG. 13 is a diagram illustrating an example receiver adapted to receivea phase modulated signal;

FIG. 14 is a diagram illustrating a first example waveform of phasemodulation added to a pulse-width amplitude modulated carrier in anexample communication protocol;

FIG. 15 is a diagram illustrating a second example phase modulationadded to a pulse-width amplitude modulated carrier in an examplecommunication protocol;

FIG. 16 is a diagram illustrating an example phase modulated carrier inan example communication protocol; and

FIG. 17 is a diagram illustrating the structure of an examplesuper-frame incorporating timing and time information.

DETAILED DESCRIPTION OF THE INVENTION

A high level block diagram illustrating an example timing and timeinformation transmitter a system operating in accordance with thepresent invention is shown in FIG. 1. The equipment at the transmitterend, generally referenced 10, comprises a high accuracy clock source(frequency source) 12 from which a clock signal (timing information) isderived, a time-code-generator 14 having user-interface 16, a source oftime data 13, a transmitter 18 generating a TX signal 19 and coupled totransmitting antenna 11.

The time code generator 14 keeps track of time based on thehigh-accuracy frequency source input to it from source 12, constructsthe frames of data representing the time information received from timedata source 13 and other information that is to be transmitted,modulates the data frames onto the RF carrier in accordance to aprotocol and allows time initialization and other controls to be set init through its user interface 16. The transmitter 18 amplifies themodulated signal to generate an output TX signal 19 at the desiredlevels, e.g., 50 kW, and drives the antenna 11 that is used for thewide-coverage omnidirectional broadcasting of the signal.

A high level block diagram illustrating an example timekeeping deviceconstructed in accordance with the present invention is shown in FIG. 2.Typically, the timekeeping device is incorporated into low cost consumermarket products, but may be implemented in any device that requires aprecision time reference. The timekeeping device, generally referenced20, comprises receiving antenna 21, receiver module 24 operative toreceive RX signal 22, processor and controller 26, timekeeping function30, internal or external clock source 31, display 32 and user interface34.

The receiver module 24 extracts timing and time information from thereceived signal 22, in accordance with the modulation scheme andprotocol in use, and provides the processing and control function 26with the extracted timing and time information. Controllerfunction/processor 26 appropriately enables/disables the operation ofthe receiver module through control line 28 such that it is limited tothe intervals of interest to minimize energy consumption in thoseapplications where it may be critical to do so (e.g., wrist watches).The timekeeping function 30 keeps track of the time based on pulsesprovided by clock source 31 having limited accuracy. Note that the clocksource 31 may comprise any suitable clock source or clock signal such asa crystal oscillator and may be provided internal to the timekeepingdevice 20 or supplied from a source external to the timekeeping device.

The timekeeping may be adjusted by the processor/controller inaccordance with an estimated drift at a specific instant, which iseither measured or calculated or a combination of the two. The displayfunction 32 may be used to display the time as well as variousindications to the user, including reception quality, estimated boundfor error in displayed time, battery status, etc. The user interfacefunction 34, based on pushbuttons, slide-switches, a touch-screen,keypad, computer interface, a combination therefrom, or any other formof human interface, may be used to set the initial time, define themaximal allowed timing error, the time-zone according to which time isto be calculated, the use of daylight saving time, etc.

In one embodiment of the invention, the timekeeping device is operativeto extract timing and time information conveyed in a broadcast signal.Timing information denotes information related to synchronization andtracking and is used, e.g., for bit and frame synchronization. Timeinformation denotes information related to the current time beingcommunicated, such as the date and the time of day (hours, minutes,etc.), as well as scheduled events, such as an upcoming DST transition,leap second, etc.

Typical available time-broadcast signals employ some form of amplitudemodulation combined with some form of pulse width modulation (PWM) tosend binary data bits. As an example consider the WWVB signal broadcastfrom Fort Collins, Colo. in the United States of America. The WWVBsignal comprises a 60 second frame consisting of 60 one second bits.Each bit, of one second duration, is sent as a pulse width modulatedsignal where carrier signal is transmitted at a low amplitude or a highamplitude for different portions of the bit. The frame also consists ofseveral marker bits spread out evenly through the frame, which serveonly to indicate timing and do not convey time information.Representations of the different possible signal waveforms transmittedby WWVB are presented below.

The existing WWVB system transmits a pulse-width modulatedamplitude-shift keyed waveform on a 60 kHz carrier. The one-secondduration ‘0’ and ‘1’ symbols are represented by a power reduction of −17dB at the start of the second for 0.2 s and 0.5 s, respectively. FIGS.3, 5, 7 show the baseband waveforms for the ‘0’ (denoted x₀(t)), ‘1’(denoted x₁(t)) and Marker (‘M’) symbols for the existing WWVB systemwhere the low portion of the symbols are reduced in power −17 dB,corresponding to an amplitude reduction to about 0.14 of the highamplitude. FIGS. 4, 6, 8 show the baseband waveforms for the ‘0’(denoted x₀(t)), ‘1’ (denoted x₁(t)) and Marker (‘M’) symbols for anexample broadcast system where the low portion of the symbols are zeroamplitude.

A diagram illustrating a first example pulse width modulated AM signalrepresenting a ‘0’ bit is shown in FIG. 3. The signal x₀(t) 40 (upperdiagram) represents the envelope or baseband waveform of a ‘0’ bit andconsists of 0.2 seconds of low amplitude carrier (e.g., 0.14 amplitude)and 0.8 seconds of high amplitude carrier. The lower diagram shows thecorresponding carrier waveform 42 of 0.2 seconds low amplitude followedby 0.8 seconds of high amplitude.

A diagram illustrating a second example pulse width modulated AM signalrepresenting a ‘0’ bit is shown in FIG. 4. The signal x₀(t) 44 (upperdiagram) represents the envelope or baseband waveform of a ‘0’ bit andconsists of 0.2 seconds of zero amplitude carrier and 0.8 seconds ofhigh amplitude carrier. The lower diagram shows the correspondingcarrier waveform 46 of 0.2 seconds zero amplitude followed by 0.8seconds of high amplitude.

A diagram illustrating a first example pulse width modulated AM signalrepresenting a ‘1’ bit is shown in FIG. 5. The signal x₁(t) 48 (upperdiagram) represents the envelope or baseband waveform of a ‘1’ bit andconsists of 0.5 seconds of low amplitude carrier (e.g., 0.14 amplitude)and 0.5 seconds of high amplitude carrier. The lower diagram shows thecorresponding carrier waveform 50 of 0.5 seconds low amplitude followedby 0.5 seconds of high amplitude.

A diagram illustrating a second example pulse width modulated AM signalrepresenting a ‘1’ bit is shown in FIG. 6. The signal x₁(t) 52 (upperdiagram) represents the envelope or baseband waveform of a ‘1’ bit andconsists of 0.5 seconds of zero amplitude carrier and 0.5 seconds ofhigh amplitude carrier. The lower diagram shows the correspondingcarrier waveform 54 of 0.5 seconds zero amplitude followed by 0.5seconds of high amplitude.

A diagram illustrating a first example pulse width modulated AM signalrepresenting a marker ‘M’ bit is shown in FIG. 7. The signal x₁(t) 56(upper diagram) represents the envelope or baseband waveform of a ‘M’bit and consists of 0.8 seconds of low amplitude carrier (e.g., 0.14amplitude) and 0.2 seconds of high amplitude carrier. The lower diagramshows the corresponding carrier waveform 58 of 0.8 seconds low amplitudefollowed by 0.2 seconds of high amplitude.

A diagram illustrating a second example pulse width modulated AM signalrepresenting a marker ‘M’ bit is shown in FIG. 8. The signal x₁(t) 60(upper diagram) represents the envelope or baseband waveform of a ‘M’bit and consists of 0.8 seconds of zero amplitude carrier and 0.2seconds of high amplitude carrier. The lower diagram shows thecorresponding carrier waveform 62 of 0.8 seconds zero amplitude followedby 0.2 seconds of high amplitude.

A diagram illustrating the structure of an example data frameincorporating timing and time information in an example communicationprotocol is shown in FIG. 9. The frame N, generally referenced 70,comprises timing data 74, time data 76 and a field of zero or moreadditional information bits 78. The N^(th) transmitted frame is precededby frame N−1 72 and followed by frame N+1 79, both of which span 60seconds and represent the minute before and the minute after frame N,respectively.

In one embodiment, the transmitted frame 70 comprises a synchronizationsequence 74 spanning m seconds, a field of information 73 spanning kseconds that precedes the synchronization sequence and a field 78spanning the remaining time 60−(m+k) seconds following thesynchronization sequence, such that the four fields together span thetotal of 60 seconds. The values of m and k are preferably fixed andtheir sum is less than 60, such that the location of the synchronizationsequence is predictable in a frame, allowing the receiver to search forit at the expected timing, while ignoring the information bits if thereis no need to receive them.

The timing data field 74 comprises a known synchronization sequence(e.g., barker code, modified barker code, pseudo random sequence, or anyother known word or bit/symbol sequence) at a known timing within theone minute frame of 60 bits that is transmitted every 60 seconds. Notethat in alternative embodiments the synchronization sequence may beplaced within the frame such that it overlaps or straddles the frame N−1before it or frame N+1 after it.

In one embodiment of the invention, phase modulation is added to anamplitude modulated carrier. A diagram illustrating phase modulationadded to an amplitude modulated carrier in an example communicationprotocol is shown in FIG. 10. This diagram describes the amplitude/pulsewidth modulation (PWM) used in the historical WWVB broadcast as well asthe phase modulation introduced in accordance with an embodiment of thepresent invention. The diagram shows the baseband representation of the‘0’ and ‘1’ symbols in both the historical WWVB modulation and in onethat is modified in accordance with an example embodiment of the presentinvention. It is noted that the enhancement in the communicationprotocol offered by the present invention, in the form of independentlydefined phase modulation and the use of a known synchronizationsequence, is not limited to the broadcast of WWVB and may be applied toother timing/time information broadcast systems such as those in othercountries around the world where similar AM/pulse-width schemes are usedor where no AM/pulse-width modulation needs to be supported, allowingfor continuous BPSK to be used.

In one embodiment, the additional phase modulation added to the signalis binary phase shift keying (BPSK) having an 180° difference in thecarrier's phase between the ‘0’ and ‘1’ symbols, also known as antipodalphase modulation or Phase Reversal Keying (PRK). Hence, the modulatedwaveforms representing these symbols may be expressed as the products ofthe sinusoidal 60 kHz carrier (in the case of WWVB) and the basebandwaveforms s₀(t)=x₀(t) (waveform 80) and s₁(t)=−x₁(t) (waveform 84),respectively, as shown in FIG. 10. Waveform 82 represents the original‘1’ symbol s₁(t)=x₁(t) that is replaced by its inverse waveform 84 inone example embodiment of the present invention. As is shown in FIG. 10,the enhanced modulation scheme can be accomplished through simple signinversion for the waveform representing the ‘1’ symbol. It is noted thatsince the existing envelope detector based receivers designed to receiveand decode the current WWVB AM/PWM based broadcast signal do notconsider the carrier's phase, they are not impacted by the modificationof phase inversion of the ‘1’ symbol.

A diagram illustrating the signal space representation of AM only and PMover AM ‘0’ and ‘1’ symbols is shown in FIG. 11. As shown in thediagram, the new pair of waveforms, x₀ (referenced 88) and −x₁(referenced 86), having the same amount of energy (corresponding totheir distances from origin), exhibit a much greater distance betweenthe ‘0’ and ‘1’ symbols (as compared to waveform pair x₀ and x₁(referenced 90), thereby allowing for more robust reception in thepresence of additive noise. Note that the existing symbols x₀ and x₁ arestrongly correlated, i.e. they have a very short distance between themin the signal space with respect to their energies.

The Euclidean distance between the two amplitude modulated waveforms x₀and x₁ is shown to be 0.47, whereas the Euclidean distance for the twophase modulated waveforms x_(o) and −x₁ increases to 1.55. Therefore,the modulation gain (denoted m_(g)) representing the power ratio bywhich the detection capability in the presence of additive noise isimproved, is given by

$\begin{matrix}\begin{matrix}{m_{g} = {20\; {\log_{10}( \frac{1.55}{0.47} )}}} \\{= {10.36\mspace{14mu} {dB}}}\end{matrix} & (1)\end{matrix}$

Thus, by simply adding such phase modulation, an order of magnitude ofimprovement may be achieved when assuming additive white Gaussian noise(AWGN). This analysis implicitly assumes that the receivers for bothschemes would be optimal, i.e. based on correlation or matchedfiltering. In practice, the BPSK receiver may be implemented digitallyin a near-optimal fashion, whereas the receivers for the existingAM/pulse-width scheme, not designed as a classicaldigital-communications system, are based on envelope detection, aspreviously noted. This adds an additional gap of 2 to 4 dB between thetwo when only AWGN is considered. In the presence of on-frequencyinterference, however, the gain offered by realizing a near-optimal BPSKreceiver may be arbitrarily higher. Furthermore, additional gains can beoffered, such as (1) through encoding of the information, (2) use of aknown synchronization sequence, and (3) extended-duration reception inthe receiver over multiple frames (i.e. superframes).

In an embodiment of the present invention, the information representedby the phase modulation in each bit is independent from that representedby the existing (legacy) AM/pulse-width modulation, such that aninverted phase would not necessarily be tied to the shorter waveform 82,represented by inverted waveform −x₁(t) 84 in FIG. 10. In an exampleembodiment, with independent data being communicated through thecarrier's phase, a phase inverted bit, which may represent a “1”, forexample, may be combined with either a “0” or a “1” in the AM/PWMsignal, resulting in the example waveform shown in FIG. 14.

The receiver extracting the information from the phase may limit thephase demodulation operation to the last 0.5 sec of each bit, where boththe “0” and “1” symbols of the AM/PWM scheme shown in this example areat high amplitude. Alternatively, in order to gain from the additionalenergy in the longer “0” pulses (0.8 sec in this example), the receivermay extend the demodulation of phase during those symbols to 0.8 secwhen the content is of the AM/PWM modulation is known to be “0”. In theexisting WWVB protocol, for example, there are several such bits fixedat “0”. Additionally, when a device operating in accordance with thepresent invention has already acquired the time and is tracking it, itsreception of the phase modulated information may consider the predicteddurations of the time-information bits as they are defined by theparticular AM/PWM protocol, thereby further optimizing reception.

Furthermore, a receiver operating in accordance with the presentinvention may also consider some or all of the energy that a transmittedbit may have in the low amplitude portion of it, if it is greater thanzero. This is to be done by weighting that portion of the signal inaccordance with the theory of matched filtering, i.e. if the loweramplitude portion is at a normalized level of 0.14, the correlationoperation in the receiver must provide it with such weighting withrespect to the weighting of 1 that is applied during the high levelduration in the receiver symbol.

In one embodiment, the receiver determines the current time inaccordance with a nonlinear function that disregards the timing and timeinformation extracted from the received frame (along with its weighting)if its distance from the local currently assumed time in the timekeepingdevice is greater than a predefined or dynamic threshold. This it toavoid incorrect timing adjustments that could be caused by erroneousreception of the timing or time information, the likelihood of whichincreases as the SINR conditions are more severe.

In one embodiment, a dynamically adaptive threshold considers theduration over which the time-keeping device has been maintaining thetime and the statistics of the time corrections applied throughout thatduration. For example, a time keeping device that has been tracking thetime for an entire year, while performing weekly timing adjustmentsaveraging 0.8 sec, with the greatest correction being below 1.5 secondsin magnitude, may act to disregard a reception instance suggesting atiming correction of 4 seconds, whereas it would have been consideredand weighted at an earlier point in time during that year.

When the time-keeping device takes into account the timing informationextracted by correlating the appropriate portion of the received signalagainst the known synchronization sequence, an example embodiment of thepresent invention may perform such an operation utilizing linearcombining wherein the coefficient applied towards the timing extractedfrom the received signal and the coefficient applied for the locallyassumed time depend on the levels of confidence in these two timingsvariables. If, for example, the reception conditions are determined tobe excessively noisy, for which the probability of inaccurate timingextraction is higher, whereas the locally assumed time is based on arelatively recent adjustment and a good record of successive timingadjustments suggests that not much drift could have been experienced upuntil the instance of the reception at question, then relatively lowweighting may be applied towards the received timing versus the locallyassumed one. If, in contrast, the received timing is accompanied by anindication of high SINR, suggesting a high probability that it isaccurate, then it may receive higher weighting compared to that of thelocally assumed timing.

In one embodiment, a time-keeping device operating in accordance withthe present invention applies non-linear logic in its reception of timeinformation when a locally assumed time is available and has beenvalidated over time. If the device attempts to extract from a receivedframe not only the timing information, for the purpose of timingadjustment, but also time information, despite such information alreadybeing available to it, then rather than computing a new time based on alinear combination of the received time and the locally assumed one, itis to select one of the two. If the locally assumed time has beenvalidated over time and the received frame is received with errors or isaccompanied by a low SINR indication, then the device may disregard theinformation extracted from the receiver. If, however, the device'sconfidence in its locally assumed time is low and the received signal isaccompanied by an indication of reliable reception, then the receivedtime may be selected, or one or more additional frames may be receivedto further increase the confidence in the received information.

In an alternative embodiment, non-antipodal phase modulation can be usedto modulate the PWM signal. For example, the magnitude of phasemodulation applied may be set at any value less than 180°, e.g., ±45°,±25°, ±13°, etc. Use of a lower value such as ±13° ensures that themodulated signal is contained within a narrow bandwidth and does notescape the narrow filtering in typical existing AM receivers, which ison the order of 10 Hz. Note that such narrowband PM is not comparable inperformance to antipodal BPSK, where the two symbols are 180° apartexhibiting a correlation factor of −1.

A diagram illustrating an example receiver incorporating both amplitudeand phase modulation receiver paths is shown in FIG. 12. In this exampleembodiment, the receiver is operative to receive both a legacy PWM/AMmodulated broadcast signal as well as a phase modulated signal which istransmitted over the legacy PWM/AM signal. The receiver, generallyreferenced 100, comprises an AM receiver block 104 and a PM receiverblock 102, both of which are connected to antenna 106 at their input andto processor 124 at their output.

Amplitude modulation receiver 104 comprises an envelope-detector-basedreceiver of the type that is typically used in consumer market RCCdevices. The AM receiver 104 comprises band pass filter (e.g., crystalfilter) 110, envelope detector 112 and threshold block 114. As shown inthis block diagram, the AM signal is converted into an analog equivalentbaseband signal by use of a conventional nonlinear envelope detector 112(similar to the diode-based circuit in traditional AM receivers). Athreshold operation 114 that follows serves to determine the middlelevel, around which the voltages below it would be converted to a logiclow level and the voltages above it to a logic high level. The digitalprocessing stage that follows this operation measures the pulsedurations and accordingly recovers the symbols (‘1’, ‘0’, or ‘marker’).Note that, with such a receiver topology, an on-frequency interferer cancause the receiver to decode that symbol incorrectly. Typically, theeffect of the interferer is greatest when the signal is at a “low”. Ifthe interferer is exactly on-frequency, however, then it has a verysignificant effect when it is out of phase and added to the high stateof the transmitted signal (e.g., the WWVB signal).

In operation of a typical envelope detector based receiver, themodulated signal input to the receiver has two different amplitudelevels with the information represented in the durations of each ofthese levels. The high/low decision is made by following the “low” and“high” levels with dedicated peak holders (with appropriatetime-constants) and deriving the middle (average) of these two. Athreshold operation (e.g., a simple comparator) is then used to createthe logic level signals for the digital stage that follows where thepulse durations are measured and the ‘1’/‘0’/‘marker’ symbol decision ismade.

The phase modulation receiver 102 comprises a demodulator 118,correlator 120 and decoder 122. In one embodiment, the PM receiver 102is operative to receive the signal broadcast from WWVB in Fort Collins,Colo. This broadcast signal adds phase modulation (PM) to the WWVBbroadcast while maintaining the existing AM code, so as not to impactthe existing time-of-day RCC devices.

A diagram illustrating an example receiver adapted to receive a phasemodulated signal is shown in FIG. 13. In one embodiment, the receiver,generally referenced 130, comprises a coherent BPSK optimal receiverthat may be implemented digitally. The PM receiver 130 comprises antenna132 coupled to analog front end (AFE) 134, low pass filter (LPF) 136,analog to digital converter (ADC) 138, mixer 140, local synthesizedcarrier (e.g., local oscillator (LO)) 146, correlator 143 and thresholddetector 144. The filtering of the signal is based on the correlationoperation which is followed by a decision that is made in the presenceof AWGN.

The bit-error-rate (BER) performance of the receiver, for a signal tonoise ratio E_(b)/N_(o), is given by

$\begin{matrix}{{BER} = {Q( \sqrt{\frac{2 \cdot E_{b}}{N_{o}}} )}} & (2)\end{matrix}$

where E_(b) is the energy per bit and N_(o) is the noise density.

The E_(b)/N_(o) ratio is equivalent to the ratio between the power ofthe signal and the power of the noise in a bandwidth that is equal tothe bit rate, i.e. E_(b)/N_(o)=SNR @ BW=R_(b), where R_(b) representsthe bit rate. The threshold decision block 144 is where the decisionsare made and the errors occur, in direct relation to the variance ofnoise, which is assumed to have Gaussian nature and equal variancesaround the ‘0’ and ‘1’ symbols. The BER may also be expressed as afunction of the distance between the symbols in the signal space, asfollows

$\begin{matrix}{{BER} = {Q( \sqrt{\frac{d^{2}}{2 \cdot N_{o}}} )}} & (3)\end{matrix}$

where Q(x) is the tail probability of the normal distribution, i.e.

$\begin{matrix}{{Q(x)} = {\frac{1}{\sqrt{2\pi}}{\int_{x}^{\infty}{{\exp ( {- \frac{u^{2}}{2}} )}\ {u}}}}} & (4)\end{matrix}$

As previously noted, the analysis presented for the improvement obtainedthrough the introduction of the phase modulation scheme assumed only thepresence of AWGN in the receiver. In the presence of radio frequencyinterference (RFI), and particularly on-frequency interference, theperformance improvement could be much more significant and stems fromthe structure of the BPSK receiver, where the demodulation is based oncorrelation.

A diagram illustrating a first example phase modulation added to anamplitude modulated carrier in an example communication protocol isshown in FIG. 14. The waveform illustrates three consecutive examplebits in the transmission as a time-domain waveform 150. The three bits152, 154 and 156 each span a duration of one second. Each of the onesecond bits is divided into a first portion 160 for which the carrierpower is low and a second portion 162 for which the carrier power ishigh. In the WWVB protocol, the information in each bit depends on thedurations of these two portions with an even 0.5/0.5 sec partitionrepresenting a “1” bit, and the uneven 0.2/0.8 sec partitionrepresenting a “0” bit. A 0.8/0.2 sec partition represents a ‘marker’bit, which may be used for timing identification, but does not carryinformation. The bits represented under the legacy PWM/AM modulation areindicated at the top portion of the diagram. For example, the threePWM/AM bits shown are “1”, “0” and “1”.

In accordance with an embodiment of the present invention, informationis added to the existing modulation using BPSK modulation. A “1” isrepresented by a carrier having an inverted phase, with the phaseinversion 158 occurring at the beginning of the bit, as shown for thethird bit 156 at t=2 sec. It is noted that the phase inversion may alsobe performed at any other instance, e.g., during the low amplitudeportion of the carrier if the receiver's phase demodulation operation islimited to the high-amplitude duration and disregards the low amplitudeportion. While the information represented by the pulse widths is shownto be “1”, “0”, “1”, the information that is sent in parallel, inaccordance with the example BPSK (or PRK) protocol of the presentinvention, is shown to be “0”, “0”, “1” (as shown along the bottomportion of the diagram). Note that there is not necessarily anyrelationship between the bit pattern transmitted using PWM/AM and thattransmitted using PM as they can be completely independent. It is notedthat the carrier frequency is not shown to scale in the figure toenhance clarity, but it is preferable for the phase transitions to occurat zero crossing instances of the carrier.

A diagram illustrating a second example phase modulation added to anamplitude modulated carrier in an example communication protocol isshown in FIG. 15. In this second example, the carrier amplitudetransmitted during the low portions of a bit is zero rather than reducedto a lower value (e.g., −17 dB or 0.14 amplitude level) as is the casein FIG. 14. As in FIG. 14, the waveform illustrates three consecutiveexample bits in the transmission as a time-domain waveform 170. Thethree bits 172, 174 and 176 each span a duration of one second. Each ofthe one second bits is divided into a first portion 178 for which thecarrier power is zero and a second portion 180 for which the carrierpower is high.

In accordance with the present invention, the modulation of informationis added to the existing modulation using BPSK modulation. A “1” isrepresented by a carrier having an inverted phase, with the phaseinversion 182 occurring at the beginning of the bit as shown for thethird bit 176 at t=2.5 sec. While the information represented by thepulse widths is shown to be “1”, “0”, “1”, the information that is sentin parallel, in accordance with the BPSK (or PRK) protocol of thepresent invention, is shown to be “0”, “0”, “1” (as shown along thebottom portion of the diagram).

Note that there is not necessarily any relationship between the bitpattern transmitted using PWM/AM and that transmitted using PM as theycan be completely independent. It is noted that the carrier frequency isnot shown to scale in the figure to enhance clarity, but it ispreferable for the phase transitions to occur at zero crossing instancesof the carrier.

A diagram illustrating an example phase modulated carrier in an examplecommunication protocol is shown in FIG. 16. In this third example, thephase modulation is not added to a PWM/AM signal but rather is sent asthe entire bit duration. The waveform illustrates three consecutiveexample bits in the transmission as a time-domain waveform 190. Thethree bits 192, 194 and 196 each span a duration of one second. Duringeach of the bits the carrier power is high. The modulation ofinformation is performed using BPSK (or PRK) modulation, in accordancewith an embodiment of the present invention. A “1” is represented by acarrier having an inverted phase, with the phase inversion 198 occurringat the beginning of the bit, as shown for the third bit 196 at t=2 sec.The information sent in accordance with the BPSK protocol of the presentinvention is shown to be “0”, “0”, “1” (as shown along the bottomportion of the diagram). It is noted that the carrier frequency is notshown to scale in the figure to enhance clarity, but it is preferablefor the phase transitions to occur at zero crossing instances of thecarrier, as may be implemented easily when a bit spans an integer numberof carrier cycles, as is the case for WWVB, where the carrier frequencyis 60 kHz (i.e. 60,000 cycles per bit).

A diagram illustrating the structure of an example super frameincorporating timing and time information is shown in FIG. 17. In analternative embodiment, information is recovered not only from the bitsof a frame, but may also be recovered by using multiple consecutiveframes making up a superframe. In this embodiment, additionalinformation may be conveyed using the superframe, or the sameinformation from each frame may be repeated to allow for improvedreception based on the accumulated energy of multiple frames.

The use of superframes can potentially improve performance of thereceiver by nearly two orders of magnitude, which may be critical in lowSNR conditions. In one embodiment, the polarity of each of theone-minute frames in an hour is modulated (e.g., differentially orotherwise) by a corresponding bit in a 60-bit hour-synchronizationsequence. The preserved consistency between the polarities of thesynchronization sequence and the information in each of one-minuteframes permits the receiver to resolve the 180-degree phase ambiguity ofBPSK reception.

By correlating against multiple consecutive synchronization sequences,the receiver can accurately adjust its timing and can then use recordeddata from an entire hour to perform long-term integration for the hourfield (i.e. soft addition). This provides an improvement in gain of 60(i.e. 18 d B), which enables operation at SNIR values well below 0 dB(when evaluated in a 1 Hz bandwidth). While the minute and parity fieldsfor the time information vary from one minute to the next in the courseof an hour, all other fields, however, remain fixed. Thus, simpleaddition can be used to increase the total amount of energy involved inthe information recovery. Since the pattern according to which theminute frame is changing is also known, it too can serve in the extendedreception operation. The receiver may determine its timing with respectto the beginning of an hour based on the identification of a portion ofthe hour-synchronization sequence (at least six bits, collected overseven minutes) with or without recovering information from the minutefields in the received frames.

With reference to FIG. 17, a frame 216 comprises a synchronizationsequence field 218, hour field 220, minute field 222 and zero or moreadditional fields 224. A superframe (e.g., superframe P 212) is definedas a set of multiple frames (e.g., 60 frames) wherein the phase of oneor more fields in each frame may be modulated to convey information on asuperframe basis. For example, additional timing information can beconveyed by modulating the phase of the synchronization sequence fieldto define a super-synchronization sequence. Each synchronizationsequence (i.e. sync seq 0, sync seq 1, . . . , sync seq 59) is assigneda particular phase wherein the pattern is known to all receivers. Thereceivers use their knowledge of the super-synchronization sequence toaid in adjusting their time to a particular minute within the hourwithout having to recover the information from the minute field. Such asuper-synchronization sequence provides additional information forreceivers to aid in acquisition and tracking at low SINR conditions.

The use of superframes provides system scalability in that it allows forreceivers experiencing different reception conditions to use thereceived signal differently. In particular, superframes (or the use of anumber of multiple frames) allow for the accumulation of received energyover multiple one-minute frames to provide for a corresponding gain inthe receiver. For example, reception for an entire hour may provide again of 60 or 18 dB with respect to reception over a single minute (i.e.a single frame).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. As numerousmodifications and changes will readily occur to those skilled in theart, it is intended that the invention not be limited to the limitednumber of embodiments described herein. Accordingly, it will beappreciated that all suitable variations, modifications and equivalentsmay be resorted to, falling within the spirit and scope of the presentinvention. The embodiments were chosen and described in order to bestexplain the principles of the invention and the practical application,and to enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A radio receiver, comprising: a receiver circuitoperative to receive a phase modulated (PM), pulse width modulation(PWM)/amplitude shift keyed (ASK) broadcast signal encoded with timingand time information, said timing information based on a knownsynchronization sequence; and a circuit operative to extract said timingand time information from the phase of said received signal.
 2. Thedevice according to claim 1, wherein said phase modulation comprisesantipodal binary phase shift keying (BPSK) modulation.
 3. The deviceaccording to claim 1, wherein said phase modulation comprisesnon-antipodal binary phase shift keying (BPSK) modulation.
 4. The deviceaccording to claim 1, wherein said phase modulation comprisesdiscontinuous binary phase shift keying (BPSK) modulation.
 5. The deviceaccording to claim 1, wherein said extraction circuit is operative tocorrelate said phase modulated received signal against a knownsynchronization sequence to recover said timing information.
 6. Thedevice according to claim 1, wherein said timing information is used todetermine bit boundaries (symbol timing) and frame timing (minuteboundaries).
 7. The device according to claim 1, wherein said circuit isoperative to limit phase demodulation to the last 0.5 second of each bitof a legacy PWM/ASK modulation scheme.
 8. The device according to claim1, wherein said circuit is operative to limit phase demodulation tothose portions of each bit that are at high amplitude in accordance witha legacy PWM/ASK modulation scheme.
 9. The device according to claim 1,wherein said circuit performs phase demodulation on the low as well ashigh portions of each bit of a legacy PWM/ASK modulation scheme.
 10. Aradio receiver method, said method comprising: receiving a phasemodulated (PM), pulse width modulated (PWM)/amplitude shift keyed (ASK)broadcast signal encoded with timing and time information, said timinginformation based on a known synchronization sequence; and extractingsaid timing and time information from the phase of said received signal.11. The method according to claim 10, wherein said phase modulationcomprises antipodal binary phase shift keying (BPSK) modulation.
 12. Themethod according to claim 10, wherein said phase modulation comprisesnon-antipodal binary phase shift keying (BPSK) modulation.
 13. Themethod according to claim 10, wherein said phase modulation comprisesdiscontinuous binary phase shift keying (BPSK) modulation.
 14. Themethod according to claim 10, wherein said phase modulated receivedsignal is correlated against a known synchronization sequence to recoversaid timing information.
 15. The method according to claim 10, whereinextracting said timing and time information comprises limiting phasedemodulation to the last 0.5 second of each bit of a legacy PWM/ASKmodulation scheme.
 16. The method according to claim 10, whereinextracting said timing and time information comprises limiting phasedemodulation to those portions of each bit that are at high amplitude inaccordance with a legacy PWM/ASK modulation scheme.
 17. The methodaccording to claim 10, wherein extracting said timing and timeinformation comprises performing phase demodulation on the low as wellas high portions of each bit of a legacy PWM/ASK modulation scheme. 18.A radio receiver method for use in a timekeeping device, said methodcomprising: receiving a phase modulated (PM), pulse width modulated(PWM)/amplitude shift keyed (ASK) broadcast signal encoded with timingand time information, said timing information based on a knownsynchronization sequence; extracting said timing and time informationfrom the phase of said received signal; and correlating said timinginformation against a known synchronization sequence so as to establishframe and symbol timing.
 19. The method according to claim 18, furthercomprising applying averaging filtering whereby the timing informationextracted from the received signal is weighted against the localcurrently assumed time in said timekeeping device.
 20. The methodaccording to claim 19, wherein said averaging and weighting are adaptiveand performed using coefficients adapted to consider the level ofconfidence in the received signal, based on the reception conditions,against the level of confidence in the current time, based on the timethat has elapsed since the last timing correction and the monitoredstatistics of historical timing adjustments.
 21. The method according toclaim 18, further comprising determining a current time as a nonlinearthreshold-comparison function of the timing and time informationextracted from a received frame, said threshold-comparison functionadapted to determine whether the distance between said extractedinformation and the local currently assumed time in said timekeepingdevice is greater than a predefined threshold.
 22. The method accordingto claim 18, wherein the time information extracted from a receivedframe is compared against local assumed time allowing the device tovalidate said local assumed time or replace it with said extracted timeinformation based on whether or not the reception conditions suggesthigh confidence in the corresponding received information.
 23. Themethod according to claim 22, wherein the time information extractedfrom a received frame is validated against successive receptionsobtained in one or more subsequent frames before being used in place ofthe local assumed time in said timekeeping device.
 24. The methodaccording to claim 18, further comprising accumulating received energyover multiple one-minute frames thereby providing a corresponding gainin reception.
 25. The method according to claim 18, further comprisingreceiving superframes consisting of sets of 60 frames used to extracthourly timing information.
 26. The method according to claim 18, whereinsynchronization sequence fields in the frames of a superframe are phasemodulated in accordance with a known pattern.
 27. A radio receivermethod, said method comprising: receiving a phase modulated (PM)broadcast signal encoded with timing and time information, wherein saidtiming and time information, intended for synchronization and timereference purposes, is conveyed in the phase of the carrier portion ofsaid broadcast signal; and extracting said timing and time informationfrom the phase of said received signal.
 28. The method according toclaim 27, wherein said phase modulation comprises antipodal binary phaseshift keying (BPSK) modulation.
 29. The method according to claim 27,wherein said phase modulation comprises non-antipodal binary phase shiftkeying (BPSK) modulation.
 30. The method according to claim 27, whereinsaid phase modulation comprises discontinuous binary phase shift keying(BPSK) modulation.